Voice Over IP

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Voice Over IP
Developing IPHONE Jeremy Stanley CS 460 section 1
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VoIP Advantages

• No long distance fees
• No dedicated circuit
• More efficient use of networks
• 64 Kbps raw audio vs. 13 Kbps compressed
• Data need not be transmitted during silence

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VoIP Issues

• Latency
• Packetization
• Network jitter
• Quality of Service
• Real-time requirements
• Addressing Issues
• IPv6 is a pratical necessity

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Voice Encoding

• PCM (pulse code modulation)
• ADPCM - adaptive delta PCM
• Fast 41 Compression, Good quality
• Vocoders (LPC, CELP, GSM)
• Slower, 12-30x compression common
• Voice only - Music does not work well

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Introduction to IPHONE

• Win32 application
• Uses Windows Multimedia and Sockets APIs for
  audio and communication
• Two-way, full duplex voice channel (PC to PC)
• Runs over UDP or TCP, supports data compression
  and silence detection

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IPHONE Design
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Screen Shot
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The Naïve Algorithm

• Receive thread
• Receive a packet
• Decode and play audio
• Return to step 1
• Send thread
• Record audio
• Encode, packetize, and send
• Return to step 1

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Problems

• Sounds like talking into a fan
• Latency quickly rises to intolerable levels

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Timing Issues
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Solutions

• Asynchronous sound processing
• Encode and send previous packet while recording
  current packet
• Receive and decode next packet while playing
  current packet
• At least two send and receive buffers required

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Revised Algorithm
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Network Jitter

• The double-buffered solution last described works
  well on a LAN
• Delay still accumulates when receive buffer
  empties due to delayed packets
• Increasing receive buffer only does so much - it
  will eventually be exhausted

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Jitter Solutions

• Drop late packets
• Imperceptibly increase playback speed
• Shorten delays between words
• Use silence detection
• IPHONE does this

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Silence Detection

• Do not send audio chunks whose maximum amplitude
  is below a silence threshold
• Phone conversations are 60 silence
• Turns voice into bursty data

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Comfort Noise

• Total silence construed as dead line
• Substitute pre-recorded "open mic"
• Play low-volume white noise
• Record and repeat periods of silence during
  conversation

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Demonstration/Questions